1. Field of the Invention
This invention generally relates to a device for developing an electrostatic latent image by applying a single component developer thereto, and particularly to a developing device suitable for use in imaging machines such as electrophotographic copiers and electrostatic recording machines for developing an electrostatic latent image by applying magnetic toner thereto in the form a thin film.
2. Description of the Prior Art
In developing an electrostatic latent image formed on the surface of a photosensitive member in the form of a belt or drum which is being driven to travel along a predetermined path by applying magnetic toner thereto, the magnetic toner is first formed into a thin film on the outer peripheral surface of a developing sleeve, which is also being driven to rotate, while being charged to a predetermined polarity and then the thin film of charged magnetic toner is applied to the latent image at a developing region where the surface of the photosensitive member comes closer to or into contact with the surface of the developing sleeve on which the thin film of charged toner is formed. In order to keep the magnetic toner magnetically attracted to the outer peripheral surface of the developing sleeve, use is typically made of a columnar magnet disposed inside of the developing sleeve. Thus, the magnetic toner is transported along a circular path as magnetically attracted to the outer peripheral surface of the sleeve while the sleeve is being driven to rotate, and the magnetic toner on the sleeve is selectively transferred to the electrostatic latent image formed on the photosensitive member electrostatically by overcoming the magnetic attractive force.
Use of such a columnar magnet as disposed inside of the developing sleeve limits miniaturization of the developing device and tends to make the entire device heavier in weight. Particularly, if a relatively high magnetic attractive force is desired at the surface of the developing sleeve, the magnet tends to become larger thereby making the entire device larger in size and heavier in weight.
In developing an electrostatic latent image using a single-component developer, difficulty is often encountered in attaining the so-called edge effect. FIG. 1 is a graph showing ideal developing characteristics normally desired for a developing device for developing an electrostatic latent image, in which the abscissa is taken for the density of an original image and the ordinate is taken for the density of a copy image which is reproduced from the original image. In FIG. 1, the solid line A indicates an ideal developing characteristic for an area image such as a picture abundant in gray-scale representation and the dotted line B indicates an ideal developing characteristic for a line image such as alphanumeric characters. The solid line curve A has a slope of approximately 45.degree. indicating that, in the case of an area image, a copy image substantially corresponds in image density to an original image; whereas, the dotted line curve B has a steeper slope, indicating that, in the case of a line image, the image density of a copy image is increased especially for an original image having a lower image density. This is based on the fact that, in the case of a line image, it is normally desired to obtain a copy image of increased image density even if its original image is rather lower in image density because a copy image of increased image density is easy to read.
In developing an electrostatic latent image with the use of the conventional two-component developer comprising toner particles and iron carrier beads, the carrier beads help obtain the above-described edge effect so that a copy image of increased density may be obtained in the case where the original is a line image as indicated by the dotted line B in FIG. 1. However, in the case of using the so-called single component developer, or magnetic toner, to develop an electrostatic latent image, such an edge effect cannot be obtained sufficiently because of absence of the iron carrier beads. Accordingly, in the case of a line image, a copy image tends to stay low in density if its original image is low in density.